A screw-and-post form of connector panel has been in use in telephone systems for quite some time. It includes a base of insulation that bears a pattern of metal posts. Below the base, each post has a "wire-wrap" terminal; and above the base, a screw is threaded into the top of the post. In using such connector panels, the end portion of each wire must first be stripped of insulation. The bared wire end is bent around a screw which is then tightened to connect the wire securely to the post. Washers under the screw head allow two wires to be connected to a single post. The screws are accessible by test probes. In addition, standardized test blocks are widely used in manufacturing plants and in the field, having a pattern of spring-loaded contact pins matching the pattern of screw terminals of the connector panels.
In widely prevalent practice, IDC connectors--insulation-displacement connectors--replace the screw-and-post connectors just described. An IDC connector for a telephone connector panel consists of a strip of metal having a wire-wrap terminal at one end and paired resilient wire-gripping fingers formed by slitting a contact strip lengthwise. The ends of the fingers form a "V". In use, an insulated wire is laid across the "V" and forced broadside between the slit edges of the resilient fingers. The fingers are stiff enough so that the inner conductor of the wire is bared where it is gripped by edges of the fingers. With careful design, the fingers are sufficiently resilient so that the insulation on the wire is crushed between the opposite slit edges of the connector; the inner conductor is gripped between the edges without exceeding the elastic limit of the metal strip. With careful design, such connectors can be made that do not cut deeply into and weaken solid wire. A slit-strip connector can be designed for both solid and stranded wire.
Slit-strip or IDC connectors are known which bear caps of insulation for driving wires into place between slit edges, in the process cutting or crushing the wire's insulation. The cap has a cross hole for admitting a wire. With the wire in place, the cap is forced down and along the slit-finger connector for driving the wire between the slit fingers. Such insulating caps are intended for labor-saving convenience. However, a panel of close-spaced IDC connectors bearing wire-driving caps as shipped to a user are not ready for wiring. In order to insert a wire into a connector near the center of the pattern, it is first necessary to depress the caps of some adjacent connectors in order to provide access to that terminal which is to be used. Later, when one of the close-spaced connectors having a depressed cap is to be wired, that cap must first be pried up. Moreover, making individual test connections to IDC connectors bearing insulation caps may be difficult; a test clip or probe may or may not be usable. However, the insulating caps rule out use of a test block having a pattern of resilient contact pins matching the pattern of slit-strip connectors of the panel.
It is possible to construct a panel of IDC slit-finger connectors having dual slits. Each pair of fingers securely grips a respective inserted wire; inserting a wire between a second pair of fingers of a dual IDC connector does not disturb a wire previously gripped by a first pair of fingers of that IDC connector. Connectors having dual slits are wider than single-slit connectors. Therefore, the pattern of connectors in a panel of single-slit connectors ordinarily differs from the pattern of dual-slit connectors, requiring two different blocks of test probes.